Rapid capture of a slipping high-mu wheel during a setpoint speed regulation operation

ABSTRACT

A method and an apparatus for regulating the speed of the low-μ wheel as a vehicle starts from rest or accelerates on a road surface having adhesive friction values that differ between the left and right sides of the vehicle (split-μ), the low-μ wheel being braked by braking intervention when it has exceeded a defined slip threshold. In order to improve the acceleration behavior of the vehicle, it is proposed to regulate the speed of the slipping low-μ wheel to a defined setpoint speed during a setpoint speed regulation phase in which the setpoint pressure for the brake of the low-μ wheel is set so that the wheel tracks the setpoint speed; to discontinue setpoint speed regulation when the high-μ wheel meets a defined breakaway condition; to initiate a setpoint pressure control operation during a setpoint pressure control phase in which the setpoint pressure of the low-μ wheel is abruptly reduced in order to decrease the drive torque transferred to the high-μ wheel; and lastly to continue setpoint speed regulation.

FIELD OF THE INVENTION

The present invention relates to a method and an apparatus forregulating the speed of the low-μ wheel as a vehicle starts from rest oraccelerates on a road surface having adhesive friction values thatdiffer between the left and right sides of the vehicle (split-μ).

BACKGROUND INFORMATION

As a vehicle starts from rest and accelerates on a split-μ surface, adrive slip control system usually intervenes in operation of the vehiclein such a way that when a defined slip threshold is exceeded the wheellocated on the slick side of the road surface (low-μ wheel) is braked bybraking intervention, and optionally the engine torque is reduced.

In the braking intervention, the braking torque exerted by the brake ofthe low-μ wheel is transferred via the differential to the other wheelthat is not yet slipping. This transferred drive torque can in turncause the wheel that is not yet slipping (high-μ wheel) also to be beginslipping; as a result, the stability and in particular lateral stabilityof the vehicle, as well as traction at the wheel which is stilladhering, are lost, and critical driving situations can occur.

To prevent detachment of the high-μ wheel on a preventive basis, inpreviously known drive slip control systems the braking pressure at thelow-μ wheel, and thus the drive torque at the high-μ wheel, are modified(modulated) only very carefully. The acceleration behavior of thevehicle thereby suffers, especially when starting from rest andaccelerating on slopes.

If slippage of the high-μ wheel nevertheless occurs, existing drive slipcontrol systems are not capable of intercepting the breakaway of thehigh-μ wheel sufficiently quickly, and rapidly re-establishing vehiclestability.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to create a methodand a regulation apparatus with which the acceleration behavior of thevehicle under split-μ conditions can be improved, the intention being toensure vehicle stability and controllability even when the high-μ wheelis breaking away.

The idea of the present invention is to regulate the speed of a slippinglow-μ wheel to a defined speed during a setpoint speed regulation phasein which the setpoint pressure for the brake is set so that the wheeltracks the setpoint speed; to discontinue setpoint speed regulation whenthe high-μ wheel meets a defined breakaway condition; and then toperform a setpoint pressure control operation in which the setpointpressure of the low-μ wheel is abruptly reduced in order to decrease thelocking torque transferred to the high-μ wheel; and lastly to continuesetpoint speed regulation. The abrupt reduction in setpoint pressure atthe low-μ wheel results in a sudden load relief on the high-μ wheel,which is therefore rapidly recaptured.

According to a preferred embodiment of the present invention, thesetpoint pressure is lowered by way of the setback of at least 10 bar,preferably at least 15 bar, and in particular 20 bar. As a result ofthis direct and rapidly controlling action on the setpoint pressure ofthe low-μ wheel, the load on the high-μ wheel that has begun to slip isabruptly relieved, and it is rapidly recaptured.

In order to improve the dynamic behavior of a hydraulic positioner (forsetting the braking pressure), a suitable filter (preferably a dtfilter) positioned after a control unit is used. For example, if thecontrol unit defines a setback of 10 bar, this is first amplified to,for example, 20 bar in order then to approach, e.g. in accordance withan e-function, the setpoint pressure outputted by the control unit.

After capture of the high-μ wheel, setpoint speed regulation iscontinued, an initial setpoint speed value preferably being set that ishigher than the setpoint speed value at the end of the first setpointspeed regulation phase.

The initial setpoint speed value is preferably at least 1 m/s, and inparticular at least 2 m/s, higher than the setpoint speed value at theend of the first setpoint speed regulation phase. As a result, thesystem deviation between the wheel speed of the low-μ wheel and thesetpoint speed after the abrupt load relief does not become too great,and excessive braking pressure is not exerted on the brake of the low-μwheel. This prevents the high-μ wheel from immediately starting to slipagain.

The magnitude of the setpoint pressure setback can be set as a functionof the wheel speed and/or the wheel acceleration of the high-μ wheel,the setback preferably becoming greater as the speed or the accelerationof the wheel increases.

According to a preferred embodiment of the present invention, in thefirst and/or second setpoint speed regulation phase, a setpoint speedprofile in the form of a straight line of negative slope is defined.

The setpoint speed profile in the second setpoint speed regulation phasepreferably has a lesser slope than the setpoint speed profile prior tothe setpoint pressure control phase. In the second setpoint speedregulation phase, the setpoint speed for the brake of the low-μ wheelwill therefore assume a lower value than in the first setpoint speedregulation phase. As a result, less locking torque is transferred to thehigh-μ wheel, which therefore exhibits less of a tendency to slip.

A breakaway condition for the high-μ wheel, beyond which the controllingintervention is performed on the setpoint pressure of the low-μ wheel,can be defined, for example, by exceedance of a slip threshold, a speedthreshold, or an acceleration threshold, optionally with additionalexpiration of a time threshold. Breakaway of the high-μ wheel isdetected, for example, when the wheel acceleration of the high-μ wheellies above a defined threshold and, for example, a defined time periodhas been exceeded, or when the slippage of the high-μ wheel lies above adefined threshold and a positive acceleration of the high-μ wheel issimultaneously ascertained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a variety of speed and pressure profiles during an ASRregulation operation.

FIG. 2 is a flow chart to explain a setpoint speed regulation actionaccording to an embodiment of the present invention.

FIG. 3 shows an ASR system that is set up for carrying out the method ofFIG. 2.

DETAILED DESCRIPTION

FIG. 1 shows a variety of speed and pressure profiles during a setpointspeed regulation operation on a low-μ wheel. The depiction representsthe regulation profile in a driving situation in which a vehicle isstarting from rest or accelerating on a road surface having differentadhesion friction values between the left and right sides of the vehicle(split-μ situation), and the low-μ wheel (curve 7) is braked by way of abraking intervention when it exceeds a defined slip threshold.

The setpoint pressure at the low-μ wheel is labeled 12, and the actualpressure 14.

FIG. 1 shows, at time t0, the initial steeply rising speed profile 7 ofa low-μ wheel that is slipping. Once a defined slip threshold has beenexceeded, setpoint braking pressure 12 of the brake of the low-μ wheelis increased in order to brake the low-μ wheel and transfer a drivetorque to the high-μ wheel which is still adhering. The control actionin this first control phase 8 is preferably a pure p-type control,implemented by way of a p-controller. As is evident, actual pressure 14acting on the brake of the low-μ wheel tracks setpoint pressure 12 witha time delay.

The low-μ wheel is braked by the increasing actual pressure 14, so thatspeed 7 reverses after reaching a peak value.

At time t1, a setpoint speed regulation action then begins, defining asetpoint speed 1 to which wheel speed 1 is regulated. What is usuallyused for this purpose is a PID controller which sets setpoint pressure12 for the brake of the low-μ wheel in such a way that the wheel trackssetpoint speed 1.

The associated ASR control system encompasses a controller, inparticular a PID controller, that regulates wheel speed 7 of the low-μwheel to a defined setpoint speed 1 during a first and second setpointspeed regulation phase 9, 11. In setpoint speed regulation phases 9, 11,setpoint pressure 12 for the brake of the low-μ wheel is set so that thewheel substantially tracks setpoint speed 1.

The manipulated variable here is setpoint pressure 12 of the low-μwheel, and the actuator preferably a continuously controllable valvewith which the braking pressure acting on the brake of the low-μ wheelcan be continuously regulated.

As is evident, at time t1 the setpoint speed 1 jumps from a passivevalue 2 to a value that corresponds substantially to the present wheelspeed, and then follows a straight line 4 of negative slope in order toslowly brake the slipping low-μ wheel. In the meantime, setpointpressure 12 moves to a relatively high value.

Shortly before time t2, the high-μ wheel also begins to slip, as isevident from speed profile 15 of the high-μ wheel. At time t2, theslipping high-μ wheel meets a breakaway condition and thereby initiatesa setpoint pressure control phase 10 in which setpoint pressure 12 ofthe low-μ wheel is abruptly (see segment 13 of setpoint pressure profile12) reduced by at least 10 bar, preferably approximately 20 bar, inorder to relieve load on the low-μ wheel and decrease the locking torquetransferred to the high-μ wheel.

In the present case, the breakaway condition is met when the wheelacceleration of the high-μ wheel is greater than a defined threshold,and when that condition persists for a defined time.

Setback 13 that is defined by the control system (e.g. 10 bar) isamplified (e.g. to 20 bar) preferably by way of a suitable filter, e.g.a dt filter. Setpoint pressure 12 is then, for example with PT1 delay,restored to the actual setback value (10 bar) of the control system,which is reached approximately at time t3. Setpoint speed 1 is heldconstant during the setpoint pressure control phase (cf. referencecharacter 5).

As is evident from speed profile 15 of the high-μ wheel, the high-μwheel is caught as a result of the sudden pressure decrease 13, and itsslip declines again relatively quickly. The low-μ wheel, on the otherhand, accelerates because of the sudden pressure decrease 13 (cf. speedprofile 7 after time t3), and therefore needs to be regulated again.

The second setpoint speed regulation phase 11 begins after capture ofthe high-μ wheel has been detected, for example because the slip, speed,or acceleration has fallen below a threshold, optionally with theadditional expiration of a time threshold.

The setpoint speed regulation action begins with an initial value thatis higher, by a value equal to a defined offset 17, than the setpointspeed value at the end of first setpoint speed regulation phase 9. Inthe present example, offset 17 is 1 m/s, and serves to keep the systemdeviation between wheel speed 7 and setpoint speed 1 from becomingexcessive.

During second setpoint speed regulation phase 11, once again a setpointspeed 1 is defined, in the form of a line 6 of negative slope, to whichthe low-μ wheel is regulated. Here again, a PID controller is usuallyused. The slope of line 6, however, is less than that of line 4 in firstsetpoint speed regulation phase 9. As a result, the controller sets alower setpoint pressure 12 than in first setpoint speed regulation phase9. The drive torque transferred to the high-μ wheel is thus also less,so that the latter exhibits less of a tendency to slip.

At time t4 the high-μ wheel once again begins to slip, but in this casedoes not meet the breakaway condition.

As is evident from vehicle speed profile 16, the vehicle then slowlybegins to move.

The proposed setpoint speed regulation operation with interveningsetpoint pressure control has the substantial advantage that wheelpressure 14 in the low-μ wheel can be modulated relatively quickly, sothat any breakaway of the high-μ wheel can be controlled out veryquickly. The acceleration behavior of the vehicle is thus substantiallyimproved.

FIG. 2 shows execution of the method in the context of a setpoint speedregulation operation with intervening setpoint pressure control, in theform of a flow chart. Step 20 firstly checks whether the lowμ wheel isexceeding a defined slip threshold sw. If Yes, speed 7 of the slippinglow-μ wheel is regulated, during setpoint speed regulation phase 9, to adefined setpoint speed 1 (cf. FIG. 1).

In step 22 the slippage of the high-μ wheel is monitored, and in step 23the setpoint speed regulation operation is discontinued if the wheelslip of the high-μ wheel exceeds a defined threshold value. A setpointpressure control operation is then performed, in which setpoint pressure12 of the low-μ wheel is abruptly reduced in order to decrease thelocking torque transferred to the high-μ wheel. Lastly, in step 24setpoint speed regulation is continued, the initial setpoint speed valueat the onset of second setpoint speed regulation phase 11 being higherthan the setpoint speed value at the end of first setpoint speedregulation phase 9.

FIG. 3 shows a TCS system that is set up for carrying out the methoddescribed above. The TCS system encompasses a regulation and controlunit 25 having a controller 27 for regulating the speed of the low-μwheel during first setpoint speed regulation phase 9, element 28 forperforming a setpoint pressure control operation during setpointpressure control phase 10, and a controller 29 for performing a setpointspeed regulation operation during second setpoint speed regulation phase11.

Regulation and control unit 25 coacts with a wheel brake 26 in order toset the defined wheel slip at the low-μ wheel. The wheel slip isdetermined from the speed v of the driven wheels.

List of reference characters  1 Setpoint speed  2 Passive value  3Setback  4 Line  5 Flat segment  6 Line  7 Wheel speed  8 P-controlphase  9 Setpoint speed regulation phase 10 Setpoint pressure controlphase 11 Second setpoint speed regulation phase 12 Setpoint pressure 13Setpoint pressure reduction 14 Actual pressure 15 Speed of high-μ wheel16 Vehicle speed 17 Offset 20-24 Method steps 25 Regulation and controlunit 26 Wheel brake 27 Controller 28 Setpoint pressure control device 29Controller t0-t4 Time values v Wheel speed

1. A method for regulating a speed of a low-μ wheel as a vehicle one of starts from rest and accelerates on a road surface having adhesive friction values that differ between a left side of the vehicle and a right side of the vehicle, comprising: braking the low-μ wheel by a braking intervention when the low-μ wheel exceeds a defined slip threshold, thereby becoming a slipping low-μ wheel; regulating the speed of the slipping low-μ wheel to a defined setpoint speed according to a setpoint speed regulation during a first setpoint speed regulation phase in which a setpoint pressure for the braking of the low-μ wheel is set so that the low-μ wheel tracks the setpoint speed; discontinuing the setpoint speed regulation when a high-μ wheel meets a defined breakaway condition; performing a setpoint pressure control operation during a setpoint pressure control phase in which the setpoint pressure of the low-μ wheel is abruptly reduced in order to decrease a drive torque transferred to the high-μ wheel; and continuing the setpoint speed regulation in a second setpoint speed regulation phase.
 2. The method as recited in claim 1, further comprising: performing a setback in the setpoint pressure of at least 10 bar.
 3. The method as recited in claim 1, further comprising: performing a setback in the setpoint pressure of at least 15 bar.
 4. The method as recited in claim 1, further comprising: defining a setback by a control system; and amplifying the setback by a dt filter.
 5. The method as recited in claim 1, wherein: an initial value of the setpoint speed at an onset of the second setpoint speed regulation phase is higher than a value of the setpoint speed at an end of the first setpoint speed regulation phase.
 6. The method as recited in claim 5, wherein: the initial value of the setpoint speed is at least 1 m/s higher than the value of the setpoint speed at the end of the first setpoint speed regulation phase.
 7. The method as recited in claim 5, wherein: the initial value of the setpoint speed is at least 2 m/s higher than the value of the setpoint speed at the end of the first setpoint speed regulation phase.
 8. The method as recited in claim 1, wherein: a magnitude of a setpoint pressure setback is accomplished as a function of one of a wheel speed and a wheel acceleration of the high-μ wheel.
 9. The method as recited in claim 1, further comprising: during the first setpoint speed regulation phase and the second setpoint speed regulation phase, defining a setpoint speed profile as a straight line of a negative slope.
 10. The method as recited in claim 9, wherein: the setpoint speed profile in the second setpoint speed regulation phase has a lesser slope than that in the first setpoint speed regulation phase.
 11. The method as recited in claim 1, further comprising: after a breakaway of the low-μ wheel, performing a p-control operation on a wheel speed of the low-μ wheel during a first regulation phase.
 12. An apparatus for regulating a speed of a low-μ wheel as a vehicle one of starts from rest and accelerates on a road surface having adhesive friction values that differ between a left side of the vehicle and a right side of the vehicle, comprising: an arrangement for braking the low-μ wheel by a braking intervention when the low-μ wheel exceeds a defined slip threshold, thereby becoming a slipping low-μ wheel; a controller for regulating the speed of the slipping low-μ wheel to a defined setpoint speed according to a setpoint speed regulation during a first setpoint speed regulation phase in which a setpoint pressure for the braking of the low-μ wheel is set so that the low-μ wheel tracks the setpoint speed; an arrangement for performing a setpoint pressure control operation during a setpoint pressure control phase in which the setpoint pressure of the low-μ wheel is abruptly reduced in order to decrease a drive torque transferred to a high-μ wheel; and a controller for performing the setpoint speed regulation in a second setpoint speed regulation phase.
 13. The apparatus as recited in claim 12, wherein: the controller for performing the setpoint speed regulation defines at a beginning of the second setpoint speed regulation phase an initial value of the setpoint speed that is higher than a value of the setpoint speed at an end of the first setpoint speed regulation phase.
 14. The apparatus as recited in claim 12, wherein: the arrangement for performing the setpoint pressure control operation includes a dt filter.
 15. The apparatus as recited in claim 12, wherein: the controller for performing the setpoint speed regulation operation includes a PID controller.
 16. The apparatus as recited in claim 12, further comprising: a continuously controllable valve as an actuator. 